Crystallizable glass and the use thereof for producing extremely solid and break resistant glass-ceramics having an easily polished surface

ABSTRACT

The invention relates to crystallizable aluminosilicate magnesium-containing glass which is used for producing extremely solid and break-resistant glass-ceramics having an easily polished surface. The inventive crystallizable glass contains 5-33 mass % of SiO 2 , 25-40 mass % of Al 2 O 3 , 5-25 mass % of MgO, 0-15 mass % of B 2 O 3 , 0.1-30 mass % of Y 2 O 3 , Ln 2 O 3 , As 2 O 3  and/or Nb 2 O 5  and 0.1-10 mass % of P 2 O 5 .

The invention relates to a highly rigid, break-resistant, crystallizableglass of the magnesium-containing aluminosilicate type and to glassceramics produced therefrom and having an easily polished surface, andto the use thereof in magnetic storage disks and mirror systems or as asubstrate therefor.

Strict requirements in terms of breaking resistance, high specificrigidity and high surface quality are placed on magnetic storage disksand magneto-optical storage materials as well as on purely opticalstorage materials. The rising requirements in terms of storage densityand access velocity placed on, for example, hard disk drives result inhigher mechanical loads on the substrate materials. To bring about adefinite reduction in access time, the rotation speed of the storagedisk must be increased to more than 15,000 rpm and, in addition, thedistance of the reading head from the disk surface must be furtherreduced. To make this possible, carrier materials are needed that have ahigh breaking resistance (Klc and flexural strength) and a very highmodulus of elasticity or a very high specific rigidity and thus a lowflutter amplitude. Moreover, it is absolutely necessary that thematerial have a very low surface roughness of Ra <0.5 nm at a wavinessof <10 nm (ISO 1305 or DIN 4768). In addition, during preparation of amagnetic coating, the substrate or carrier material must withstandthermal loads in the range of about 400-450° C. and, because ofpronounced temperature changes such as those occurring, for example, insputter processes, must be resistant to temperature change. Finally, thethermal expansion of the storage materials and mirrors must be adaptedto the recording device (spindles and spacers). These are currently madeof steel so that a thermal expansion coefficient α₂₀₋₃₀₀ of about 12ppm/K is optimal, although lower values are also tolerable.

Currently used substrates for magnetic storage disks are made ofaluminum alloys, glasses and glass ceramics. Although glasses have ahigher modulus of elasticity, they have the drawback of a low Klc value.By thermal or chemical hardening, this value can be improved but only toa limited extent.

Because of their heterogeneous structure resulting from microcrystalsembedded in a glass matrix, glass ceramics cannot be polished as well asglass itself or aluminum. Until now, glass ceramics therefore have onlyrarely reached the required surface roughness values of Ra <0.5 nm. Thisis due to the crystallites located near the surface which are generallyharder than the glass phase that surrounds them. During the polishingsteps therefore, more material is removed from the glass than from thecrystallites resulting in a rough surface. For many applications, suchmaterials are therefore unsuitable.

Glass ceramics, also known as vitroceramics, are polycrystalline solidsprepared by selective devitrification, namely by crystallization fromglasses that are particularly well suited for this purpose. Thiscrystallization or ceramization is achieved by heating the glass objectsor optionally by irradiation. As a result, however, the glass ceramicmaterials still contain a residual amount of a glass-phase matrix inwhich the crystals are embedded. Because by conventional glass-shapingtechniques any desired shape can be imparted to glass ceramics in theirinitial glassy stage and because glass ceramics have many desirableproperties such as resistance to temperature variation, low expansioncoefficient and good electric insulation, they are suitable for thefabrication of many objects, for example hobs, cooking utensils,high-tension insulators, laboratory equipment and bone replacements orfor sealing off environmental pollutants, for example spent nuclear fuelrods.

A well researched system for making glass or glass ceramics is thethree-component SiO₂—Al₂O₃—MgO system (MAS system). This three-componentsystem has several composition regions wherein there exist or are stableor form differently specific crystalline phases. Until now, thedescriptions of glass ceramics in the literature were limited to thoseregions of the MAS system in which the crystal phases quartz (SiO₂),tridymite (SiO₂), enstatite (MgO.SiO₂), cordierite (2 MgO.2 Al₂O₃.5SiO₂), forsterite (MgO.SiO₂), mullite (3 Al₂O₃.2 SiO₂) and possiblyspinel (MgO.Al₂O₃) exist as the thermodynamically most stable phases andthus could be referred to as the main crystalline phase.

The relatively narrow region in which stable glasses are known to existhave repeatedly been described in the literature, for example in P. W.McMillen: “Glass Ceramics”, Academic Press, London, NY, San Francisco,2nd ed. (1979), pages 18 ff. There it is also stated that TiO₂, ZrO₂ andP₂O₅ can be used as nucleation agents for the conversion of glasses ofthe MAS system into glass ceramics.

U.S. Pat. No. 2,920,971 (to Stookey et al.) describes aluminosilicateglasses containing titanium oxide and magnesium oxide. In this case,thermal post-treatment brings about the precipitation of cordierite asthe crystalline magnesium aluminum silicate phase.

EP-A-0 289 903 describes a glass-coated/ceramic-coated substratecomposition of the afore-said three-component system which contains42-68 wt. % of SiO₂.

JA-91045027 B (to Nishigaki, J., et al.), JA-91131546 A (to Tanabe, N.,et al.), JA-92106806 A (to Okubo, F., et al.) and EP 55 237 7 (toKawamura et al.) describe different glass or glass ceramic compositions.These compositions, however, contain no crystalline magnesium aluminumsilicate phases or they have a SiO₂content of less than 33 wt. %.

EP-A-1 067 101, EP-A-1 067 102 and EP-A-0 941 973 describeyttrium-containing MAS glass ceramics as substrates for storage media.In these documents it is stated that the addition of 0.8-10 mol % ofyttrium oxide to a basic glass mixture consisting of 35-65 mol % ofSiO₂, 5-25 mol % of Al₂O₃, 10-40 mol % of MgO and 5-12 mol % of TiO₂causes these glasses to melt more readily, to exhibit good mechanicalproperties and, after heat treatment, to give a glass ceramic with amodulus of elasticity of >130 GPa. These ceramics contain as crystallinephases mixed crystals of high quartz of varying composition, for exampleMgO:Al₂O₃:SiO₂=2:2:5 or 1:1:3 or 1:1:4 or mixtures, as well as enstatite(MgO.Al₂O₃ or MgO.0.5 Al₂O₃.SiO₂). The nucleation agent used here isTiO₂ which, moreover, within limits compensates for a loss oftransparency. Y₂O₃ is used as an additive to reduce the processingtemperature. A Y₂O₃ content of >10 mol %, however, is undesirable,because it causes a marked increase in the tendency of the glass tocrystallize.

The until now common glass ceramics usually contain as the maincrystalline phases enstatite, forsterite and cordierite. Spinel andsapphirine phases are referred to as secondary phases. Here the lowerlimit of the SiO₂ content is 35 wt. %, lower limits of 40 or 42-44 wt. %being common. Until now it has been assumed that, no industriallyprocessable glasses can be prepared below this SiO₂ concentration.

JP-A-2000-327365 refers to 25 wt. % of SiO₂ as the lower limit foralkali-containing glasses, and JP-A-11079785 to 30 wt. % for alkali-freeglasses.

The object of the invention is to provide novel glasses that have a lowSiO₂ content but are still industrially processable and that can beconverted into glass ceramics having a high modulus of elasticity.

Another object of the invention is to provide glass ceramics that can bepolished to the desired surface roughness and that can be used assubstrates for magnetic storage disks or mirror systems. This objectiveis reached by means of the glass defined in the claims and of glassceramics that can be obtained therefrom, as well as by the use thereof.

Surprisingly, we have now found that it is possible to produce glassesand glass ceramics containing a small amount of network-forming SiO₂below the afore-indicated range of >30 wt. % and which when Y₂O₃, Nb₂O₅and/or Ln₂O₃ is added to this glass are also suitable for industrialprocessing. In this regard, we have found, surprisingly, that such aglass is not only highly rigid and break-resistant, but that even beforethe selective nucleation or ceramization it is stable in terms of theformation of crystalline phases, namely that it can be cooled forannealing purposes. Moreover, such a glass ceramic can be polished tothe desired surface roughness of Ra <0.5 nm.

The glass of the invention or the glass ceramic obtained therefrom isformed from the “SiO₂—MgO—Al₂O₃” three-component system and additionallycontains some B₂O₃. The minimum amount of SiO₂ is 5 wt. % andparticularly 10 wt. %, with 15 wt. % being especially preferred. Theupper limit is usually 33 wt. % or 30 wt. %, with 28 wt. % andparticularly 25 wt. % being preferred.

The minimum amount of MgO is 5 wt. %, preferably 8 wt. %, with 10 wt. %being particularly preferred. The upper limit of MgO lies at 25 wt. %,with 20 wt. % being preferred. The Al₂O₃ content is at least 25 wt. %and preferably at least 30 wt. %. The maximum content of Al₂O₃ is 40 wt.% and preferably 38 wt. %. Boron oxide does not necessarily have to bepresent, but the B₂O₃ content is in many cases at least 1 wt. %, usuallyat least 2 wt. % and preferably at least 3 wt. %, the upper limit ofB₂O₃ in the composition of the invention being at the most 15 wt. %,usually at the most 12 wt. % and preferably at the most 10 wt. % or atthe most 9 wt. %.

The oxides of the group consisting of Y₂O₃, Ln₂O₃ and Nb₂O₅ are presentin the composition of the invention in an amount of at least 0.1 wt. %,usually at least 3 wt. % and preferably at least 12 wt. %. The upperlimit for these oxides is 30 wt. % and preferably 28 wt. %, an upperlimit of 25 wt. % being particularly preferred. The amounts of theindividual oxides are usually 0.1-30 wt. %, preferably 10-30 wt. %, forY₂O₃ and 0-20 wt. % for Ln₂O₃. Ln comprises the lanthanoids,particularly La, Ce, Pr, Nd, Eu, Yb, Ho and Er. The composition of theinvention can contain as additional components the common refiningagents and fluxes such as Sb₂O₃, As₂O₃ or SnO₂ in amounts commonly usedfor these purposes. The upper limit for each of Sb₂O₃ and As₂O₃ is 5%maximum and preferably 2% maximum.

In a preferred embodiment, the glass or glass ceramic of the inventioncontains 0-12 wt. % of TiO₂, 0-10 wt. % of ZrO₂, 0-5 wt. % of CaO, 0-5wt. % of SrO, 0-5 wt. % of BaO and 0-20 wt. % of ZnO. In an embodimentpreferred according to the invention, the composition contains at least2 wt. % and preferably at least 4 wt. % of TiO₂ and a maximum amount ofpreferably at the most 12 wt. % and particularly at the most 10 wt. %.To the extent that the other oxides are at all present, the minimumamount of said other oxides, namely ZrO₂ and ZnO, is usually 1 or 2 wt.% and the maximum amount at the most 5 or 8 wt. %, each.

The glass of the invention or the glass ceramic of the invention ispreferably essentially free of alkali metal oxides such as Li₂O, Na₂Oand K₂O and contains them only as impurities introduced with the othercompositions of the mixture. By “essentially alkali-free” is meant anamount of at the most 2 wt. %, an amount of at the most 0.5 wt. % beingcommon.

We have found that the glass or glass ceramic of the invention cancontain up to 10 wt. % and usually <5 wt. % of transition metal oxideswithout this causing a significant change in the resulting propertiessuch as rigidity, breaking strength and crystallization charateristics.The usual transition metal oxides present in the glass or glass ceramicof the invention comprise the oxides of the elements Fe, Co, Ni, Cr, Mn,Mo, V, Pt, Pd, Rh, Ru and W and are in particular MnO₂, Fe₂O₃, NiO, CoO,Cr₂O₃, V₂O₅, MoO₃ and/or WO₃. In an embodiment preferred according tothe invention, the sum of the components SrO, BaO and CaO is at least 1wt. %, preferably at least 2 wt. %, usually at the most 5 wt. % andparticularly at the most 4 wt. %. If present, the oxides TiO₂ and ZrO₂are present in an embodiment preferred according to the invention in anamount of at least 1 wt. %, preferably at least 2 wt. %, more preferablyat the most 13 wt. % and particularly at the most 10 wt. %.

The glass of the invention or the glass ceramic of the invention has ahigh modulus of elasticity of at least >110 GPa. Usually, the modulus ofelasticity is above 120 GPa. Depending on the ceramization program, itis possible to prepare glass ceramics with a modulus of elasticitygreater than 150 GPa, and in some cases even >200 GPa. (Determination ofthe modulus of elasticity in accordance with DIN EN 843-2, item 4,method A: static flexing method).

In the glass ceramic of the invention, the crystallites are embedded ina glassy matrix and their size is usually, but not necessarily, from<100 nm to about 3 μm. For good polishability of the glass ceramics,crystallite sizes in the range of 50-500 nm are particularly preferred.We have found that crystallization of a glass composition according tothe invention gives a glass ceramic containing as its main crystalphases spinel, sapphirine and/or cordierite. In this respect, we havesurprisingly also found that the desired properties of the glass ceramicare obtained especially when the crystal phases usually associated withhigh values of the modulus of elasticity, namely enstatite, high quartzor low quartz or mixed crystals of high quartz, are avoided, which ispossible particularly with the composition of the invention. Moreover,the glass ceramics obtained according to the invention can containcrystals with a structure of pyrochlore, A₂B₂O₇, wherein A³⁺ denotes alantanoid and/or yttrium and B⁴⁺ denotes Zr, Ti, Sn and/or Ru. Moreover,they can contain pyrosilicates having the general formula A₂Si₂O₇wherein A⁺³ denotes a lantanoid, Y and/or Sc. Preferably, however, theyare Y₂Si₂O₇ (yttrium pyrosilicate, yttrialite) or Y₂Ti₂O₇(yttropyrochlore).

According to the invention, we have also found that the order in whichthe crystal phases precipitate has a decisive influence on the modulusof elasticity. We found that after the primary precipitation of smallspinel crystallites and possibly of small sapphirine crystallites,particularly those of the Mg₂Al₄SiO₁₀ type, the subsequent secondarycrystal phases of the sapphirine and cordierite type are formed aroundthe primary crystallites, particularly as a coating over the primarycrystallites. According to the invention, we have found that the SiO₂content and the crystal structure of the secondarily precipitated phasedepend on the silicon and yttrium content of the base glass, a low SiO₂content of the base glass promoting the formation of sapphirine. By theselection of the kind and amount of nucleating agents (TiO₂, ZrO₂,P₂O₅), the size of the crystallites of the primary crystals or of thenuclei can be selectively controlled. The size of the crystallites ofthe secondary phases can be controlled kinetically or thermodynamically(utilization of diffusion and epitaxy phenomena). Present as tertiarycrystal phases are pyrochlores, pyrosilicates, xenotimes and/or rutile.By their precipitation, it is possible to influence the amount ofresidual glass phase and thus also the modulus of elasticity of theresulting glass ceramic. According to the invention, we have also foundthat in the glass ceramic of the invention TiO₂ not only acts as anucleating agent, but it also becomes incorporated into the crystalphases with a high modulus of elasticity. Surprisingly, we have alsofound that in the procedure according to the invention refining agentssuch as SnO₂ and As₂O₃ become integrated into the spinel or pyrochlorephases. According to the invention, it is possible in this manner toreduce the amount of residual glass phases even further and at the sametime to cause selectively the precipitation of crystallites with a highmodulus of elasticity.

Because the described melts are practically alkali-free, corrosion ofthe magnetic or magneto-optical or optical layer applied to the storagesubstrate as a result of alkali diffusion is also not possible.

According to the invention, we have also found that with the glass ofthe invention a glassy layer is formed on the surface of the glassceramic object during ceramization, the thickness of said layer beingmarkedly greater than that of the amount of residual glass remainingin-between the crystallites. As a result of this glassy layer,semifinished products for storage substrates have a very low surfaceroughness. Because this glass phase can be polished better than theprecipitated crystals, the expense for subsequent processing is markedlyreduced.

In addition, the glass of the invention or the glass ceramic of theinvention has very good mechanical properties such as a high flexuralstrength of >150 MPa (determined as 3-point flexural strength inaccordance with DIN EN 843-1) and particularly >180 MPa, and a Klc of1.3 MPam^(1/2) [determined by the method of A. G. Evans, E. A. Charles,J. Amer. Ceram. Soc. 59 (1976), 371].

The glasses according to the invention are converted to thecorresponding glass ceramics by heat treatment at a temperature abovethe Tg. To this end, the conversion temperature and the formation of thecrystal phases are determined by known methods, for example with the aidof a holding curve obtained by differential thermal analysis (DTA).

To convert the glass into a glass ceramic, the glass is heated at theconversion temperature until the crystalline phases have precipitated.The glasses are usually heated at a temperature of about 5-50° C. abovethe Tg, and preferably 10-30° C. above the Tg, until the primarycrystallites have formed in sufficient quantity. The glass transitiontemperature of these glasses is usually 700-850° C.

The holding time for the formation of the primary crystallites orcrystal nuclei depends on the desired properties and usually amounts toat least 0.5 hour, preferably at least 1 hour, a length of time of 1.5hours being particularly preferred. The maximum time is usuallyconsidered to be 3 days, but 2 days and particularly 1 day are preferredas the maximum time for forming the primary crystal nuclei. In mostcases, a 2-12 hour period is sufficient. The material is then heated toa higher temperature at which the main crystal phases precipitate.

This temperature is usually at least 20° C., and preferably at least 50°C., above the temperature of formation of the primary crystallites. Inspecial cases, it was found to be advantageous, after the precipitationof the main crystal phases (secondary crystals), particularly of spinel,sapphirine and/or cordierite, to heat the material once again to anotherhigher temperature to cause the precipitation, from the residual glassphase remaining in between the primary and/or secondary crystals, ofother crystal phases, for example of pyrochlores, pyrosilicates,xenotimes and/or rutile as well as of mixture thereof.

The glass ceramic of the invention has a thermal expansion coefficient(TEC) α₂₀₋₆₀₀ of 4-9×10⁻⁶ K⁻¹ (determined in accordance with DIN-ISO7991).

The glass according to the invention is particularly well suited for thefabrication of magnetic storage disks, magneto-optical memory devices,mirror carriers or substrates therefor.

In the following, the invention will be described in greater detail.

FIG. 1 shows the results of a study of the glass of the invention bydifferential thermal analysis (DTA curve for exemplary embodiment No. 1)

To obtain the temperature—time program for the conversion according tothe invention of the base glass into a glass ceramic, the formationtemperatures of the individual crystal phases were estimated. This wasdone with the aid of differential thermal analysis. In this manner, acurve was obtained (see FIG. 1) in which the exothermic reactions areindicated as a peak (maximum) or the endothermic reactions as a dip(minimum) relative to a standard curve (dash-dot line). Crystallizationreactions are generally exothermic; changes in structure or in the stateof aggregation are usually endothermic.

For the glasses of the invention, a first minimum was obtained in thetemperature range of >700° C. and often above 740° C. The point ofinflection for the DTA curve descending toward this minimum indicatesthe transition temperature of the glass, Tg (in FIG. 1: about 780° C.).

The flat maximum in the temperature range marked 1 reflects thetemperature range of nucleation or of precipitation of primary crystalphases. In the case of the glasses/glass ceramics of the invention, inthis range takes place the precipitation of nuclei or very small spinelcrystallites that cannot be characterized more closely by analysis ofthe crystal structure (crystallite volume <150 nm³).

The temperature range marked 2 contains a clearly defined peak. Thisindicates the exothermic crystallization reaction of secondary crystalphases to primary nuclei.

In temperature range 3, exothermic reactions are also indicated bydiverse peaks attributable to the crystallization of tertiary crystalphases.

In peak-free or dip-free temperature interval 4 a ripening, growth orpossibly intrinsic recrystallization of the precipitated phases takesplace. Such processes, however, are also possible in the entiretemperature range>Tg, and thus also in temperature intervals 1, 2 and 3.

A sharp dip (in FIG. 1: about 1415° C.) marked with Fp identifies themelting point of the glass ceramic.

To prepare the glass ceramics of the invention, the nuclei or primarycrystallites were preferably formed at a temperature below two thirds ofthe temperature interval 1, it being preferred to select a temperaturewithin the lower half. Even more preferred is the selection of atemperature in the lower third of this range marked 1. At the end of asufficiently long holding time, or after the formation of a sufficientlylarge number of primary crystallites or nuclei, the material was heatedto a higher temperature at which the main crystal phases of the glassceramic material precipitate or the primary crystallites showconsiderable growth. Such a temperature usually lies in the temperatureinterval marked 2 and is at least 20 K and preferably at least 50 Kabove the nucleation temperature, a temperature range of ±50 K aroundthe peak maximum (in range 2 marked in FIG. 1) being desirable. Theglass ceramic was left at this temperature until the precipitatedcrystallites had attained a sufficient size.

The material was then heated to another higher temperature, usually fromthe temperature intervals 3 and 4. The glass ceramic was kept at thistemperature to enable the crystallization of the tertiary crystal phaseswith sufficient crystallite size.

The holding times at the particular temperatures to form the primary,secondary or tertiary crystalline phases depend on the growth velocityof these phases and usually amount to at least 15 minutes and preferablyat least 30 minutes, a holding time between 60 and 180 minutes andparticularly between 90 and 120 minutes being particularly preferred.The upper limit of the holding times is usually a maximum of 60 hoursand preferably a maximum of 12 hours. In many cases it is also possible,after the formation and ripening of the primary crystal phases ornuclei, to heat the material to a single higher temperature, for examplewithin temperature range 4 of FIG. 1, to cause at this temperature thesimultaneous crystallization or recrystallization of secondary andtertiary crystal phases.

According to the invention, during the ceramization of the startingglass, the heating to a temperature just below the Tg is carried outrelatively rapidly, namely at 5-15 K min⁻¹ and particularly at about 10K min⁻¹. The heating to the temperature that brings about theprecipitation of primary crystal phases or nuclei is then carried outmore slowly, at about 3-8 K min⁻¹ and usually at about 5 K min⁻¹. Inmany cases, the heating rate can also amount to 0.5-3 K min⁻¹. Thehigher temperatures at which secondary or tertiary crystal phasescrystallize can be attained at very different heating rates in the rangeof 0.5-200 K min⁻¹. The selection of these heating rates depends on thegrowth rates of the particular crystal phases in the matrix material inquestion.

The glasses indicated hereinbelow were prepared as follows.

In a Pt/Rh crucible at 1600-1700° C., charges of 100 g to 3 kg of aparticular glass lot were melted and cast to form plates (thickness:0.5-3 cm ). These glass plates were annealed at a temperature of Tg+20 Kand then slowly cooled to room temperature.

To prepare the glass ceramics, the glasses were heat-treated by theafore-described procedure, as indicated in the following table. Thiscaused precipitation of the crystallites of the various crystal phases.The crystallization was carried out by use of a one-step or multistepcooling program. To this end, the indication of, for example, 800° C./2h, 950° C./1 h, 1050° C./1 h, means that the glass was subjected to aheat treatment at 800° C. for 2 hours, then at 950° C. for 1 hour andfinally at 1050° C. for 1 hour. Spinel was found to form as the firstcrystalline phase, its formation taking place in the temperature rangeof about 750-900° C. after 1-2 hours. The crystal growth of the spinelor the precipitation of sapphirine or of other crystal phases wasachieved in about 2 hours in a second step of the heat treatment between850° C. and 1050° C. In some cases, crystal growth was also broughtabout by extending the holding time at a temperature around 900° C. Inthe table, Sp stands for spinel, Sa for sapphirine, Co for cordierite,Ps for yttrium pyrosilicate, Pc for yttrium pyrochlore, Xe for yttriumphosphate (xenotime) and Ru for rutile (TiO₂).

The glasses and glass ceramics prepared were comprehensivelycharacterized. The modulus of elasticity and the flexural strength weredetermined from flexural tests, the Klc value was calculated bymeasuring the radial crack lengths by the VICKERS method. The densitywas determined by the buoyancy method and the ther-mal expansioncoefficient by dilatometric measurements. The analysis of the crystalphases was performed by x-ray diffractometry. Crystal structures andtexture were derived from scanning electron micrographs. To this end,after standard polishing, scanning force microscopic studies (AFM) werecarried out to obtain a surface topography. An averaging of the measureddata gave the indicated values of the surface roughness. In this regard,Ra means the arithmetic mean and rq (or rms) the geometric mean of themeasured data. PV indicates the distance from peak to valley of themaxima/minima along a measured section.

Practical Examples P 1853 1 2 3 Wt. % Wt. % Wt. % SiO₂ 32.72 23.88 22.89B₂O₃ 3.79 8.29 3.97 P₂O₅ 0.10 0.10 8.10 Al₂O₃ 37.02 30.38 29.12 TiO₂2.90 5.57 5.34 Y₂O₃ 8.20 11.20 10.74 MgO 15.37 12.81 12.28 CaO 2.78 2.67SrO 2.06 1.97 BaO 3.04 2.92 Crystal- Modulus of Crystal- Modulus ofCrystal- Modulus of lization Elasticity Crystal lization ElasticityCrystal lization Elasticity Crystal Program (GPa) Phases Program (GPa)Phases Program (GPa) Phases glassy  90 ± 6 — glassy 117 ± 4 — glassy 109± 3 — 850° C./2 h 134 ± 4 Sp/Sa 760° C./2 h  116 ± 18 Sa/Ps 800° C./2 h 125 ± 10 Sp/Sa   930° C./0.5 h 900° C./1 h 850° C./1 h 147 ± 4Sp(?)/Sa/(Co?) 760° C./4 h 124 ± 9 Sa/Ps/Pc 760° C./4 h 122 ± 9 Sp/Sa/Xe950° C./1 h 1040° C./1 h    960° C./0.5 h 800° C./1 h Sa/Co/Ps/Pc 760°C./4 h 127 ± 7 Sp/Sa/Xe/Ru 950° C./1 h 1040° C./1 h  1050° C./1 h  1050°C./1 h  Sa/Co/Pc 1150° C./1 h  4 5 6 Wt. % Wt. % Wt. % SiO₂ 22.47 21.3723.26 B₂O₃ 3.47 1.77 1.80 P₂O₅ 7.08 5.41 7.93 Al₂O₃ 33.05 33.67 34.20TiO₂ 3.98 10.14 6.24 Y₂O₃ 16.69 14.34 11.65 MgO 13.06 13.31 13.52 CaOSrO BaO Crystal- Crystal- Crystal- Modulus of lization Crystal lizationModulus of Crystal ization Elasticity Crystal Program Phases ProgramElasticity Phases Program (GPa) Phases glassy — glassy 146 ± 4 (Sp)glassy — 1000° C./1 h Sa/Co/Xe 800° C./12 h 125 (Sp?)/Sa 600° C./60 h158 Sa 760° C./4 h  1040° C./1 h  Sa/Xe/Pc/Ru 7 8 Wt. % Wt. % SiO₂ 21.2322.70 B₂O₃ 0.00 1.77 P₂O₅ 10.03 7.20 Al₂O₃ 26.41 31.70 TiO₂ 9.41 8.11Y₂O₃ 18.61 14.00 MgO 9.97 13.10 CaO 1.32 1.42 SrO 1.22 BaO 1.81 Crystal-Modulus of Crystal- Modulus of lization Elasticity Crystal lizationElasticity Crystal Program (GPa) Phases Program (GPa) Phases glassy —glassy 135 — 800° C./12 h 125 (Sp?)/Sa 800° C./12 h 137 Sp/Sa 800° C./60h 134 Sa 800° C./48 h 148 Sp/Sa 760° C./4 h  Sa/Xe/Pc/Ru 800° C./12 h180 Sa/Xe/Pc/Ru 1040° C./1 h  1040° C./1 h  Abbr.: Sp: spinel; Sa:sapphirine; Co: cordierite; Pa: yttrium pyrosilicate; Pc: yttriumpyrochlore; Xe: yttrium phosphate (xenotime); Ru: rutile (TiO₂) ( ):phase of secondary importance ?: phase not clearly identified

1. A crystallizable magnesium-containing aluminosilicate glass formaking a highly rigid, break-resistant glass ceramic, saidmagnesium-containing aluminosilicate glass having a composition inpercent by weight, based on oxide content, comprising: SiO₂ 5 to 33Al₂O₃ 25 to 40 MgO 5 to 25 B₂O₃ 0 to 9 P₂O₅ 0.1 to 10, and at least oneof Y₂O₃, Ln₂O₃, As₂O₃, and Nb₂O₅, in which each of said Y₂O₃, saidLn₂O₃, said As₂O₃, and said Nb₂O₅ that is present in the glass ispresent in an amount of at least 0.1 percent by weight but no more than30 percent by weight.
 2. The crystallizable magnesium-containingaluminosilicate glass as defined in claim 1, containing from 10 to 30percent by weight of said Y₂O₃ and from 0 to 20 percent by weight ofsaid Ln₂O₃.
 3. The crystallizable magnesium-containing aluminosilicateglass as defined in claim 1, containing from 2 to 12 percent by weightof TiO₂, from 1 to 10 percent by weight of ZrO₂, and/or from 1 to 20percent by weight of ZnO.
 4. The crystallizable magnesium-containingaluminosilicate glass as defined in claim 3, containing from 2 to 10percent by weight of said TiO₂.
 5. The crystallizablemagnesium-containing aluminosilicate glass as defined in claim 1,containing at most 2 percent by weight of alkali metal oxides.
 6. Thecrystallizable magnesium-containing aluminosilicate glass as defined inclaim 1, containing from 0 to 5 percent by weight of CaO, from 0 to 5percent by weight of SrO, and/or from 0 to 5 percent by weight of BaO.7. The crystallizable magnesium-containing aluminosilicate glass asdefined in claim 1, containing at most 10 percent by weight of at leastone transition metal oxide.
 8. The crystallizable magnesium-containingaluminosilicate glass as defined in claim 7, in which said at least onetransition metal oxide is selected from the group consisting of MnO₂,Fe₂O₃, NiO, CoO, Cr₂O₃, V₂O₅, MoO₃ and Wa₃.
 9. The crystallizablemagnesium-containing aluminosilicate glass as defined in claim 1, whichis made by a method comprising annealing at a temperature that is 5° C.to 50° C. above Tg for two minutes to one hour.
 10. A glass ceramichaving a modulus of elasticity of more than 110 Gpa and made by a methodcomprising heating a crystallizable magnesium-containing aluminosilicateglass above a Tg thereof, said crystallizable magnesium-containingaluminosilicate glass having a composition in percent by weight, basedon oxide content, comprising: SiO₂ 5 to 33 Al₂O₃ 25 to 40 MgO 5 to 25B₂O₃ 0 to 9 P₂O₅ 0.1 to 10, and at least one of Y₂O₃, Ln₂O₃, As₂O₃, andNb₂O₅, in which each of said Y₂O₃, said Ln₂O₃, said As₂O₃, and saidNb₂O₅ that is present in the glass is present in an amount of at least0.1 percent by weight but no more than 30 percent by weight.
 11. Theglass ceramic as defined in claim 10, in which said crystallizablemagnesium-containing aluminosilicate glass contains from 10 to 30percent by weight of said Y₂O₃ and from 0 to 20 percent by weight ofsaid Ln₂O₃.
 12. The glass ceramic as defined in claim 10, in which saidcrystallizable magnesium-containing aluminosilicate glass contains from2 to 12 percent by weight of TiO₂ and/or from 1 to 10 percent by weightof ZrO₂.
 13. The glass ceramic as defined in claim 10, in which saidcrystallizable magnesium-containing aluminosilicate glass contains atmost 2 percent by weight of alkali metal oxides.
 14. The glass ceramicas defined in claim 10, in which said crystallizablemagnesium-containing aluminosilicate glass contains from 0 to 5 percentby weight of CaO, from 0 to 5 percent by weight of SrO, and/or from 0 to5 percent by weight of BaO.
 15. The glass ceramic as defined in claim10, in which said crystallizable magnesium-containing aluminosilicateglass contains less than 5 percent by weight of at least one transitionmetal oxide selected from the group consisting of MnO₂, Fe₂O₃, NiO, CoO,Cr₂O₃, V₂O₅, MoO₃ and WO₃.
 16. The glass ceramic as defined in claim 10,having a thermal expansion coefficient (α₂₀₋₆₀₀) of 4 to 9×10⁻⁶ K⁻¹, aflexural strength of greater than 150 MPa, a surface roughness Ra ofless than 0.5 nm, and a Klc of 1.3 M Pam^(1/2).
 17. A method of making aglass ceramic, said method comprising the steps of: a) providing acrystallizable magnesium-containing aluminosilicate glass with acomposition in percent by weight, based on oxide content, comprising:SiO₂ 5 to 33 Al₂O₃ 25 to 40 MgO 5 to 25 B₂O₃ 0 to 9 P₂O₅ 0.1 to 10, andat least one of Y₂O₃, Ln₂O₃, As₂O₃, and Nb₂O₅, in which each of saidY₂O₃, said Ln₂O₃, said As₂O₃ and said Nb₂O₅ that is present in the glassis present in an amount of at least 0.1 percent by weight but no morethan 30 percent by weight; b) heating said crystallizablemagnesium-containing aluminosilicate glass to a first nucleationtemperature within a first temperature interval above Tg of saidcrystallizable magnesium-containing aluminosilicate glass in order toform primary crystallites or a primary crystalline phase of spinel andsapphirine; c) after a holding time of at least 30 minutes in which theheating of step b) takes place, heating to a main crystallizationtemperature within a second temperature interval above the firsttemperature interval in order to precipitate and grow secondarycrystalline phases of sapphirine and cordierite and then optionallyheating to another higher temperature to precipitate and grow othercrystalline phases of the xenotime, yttrium pyrosilicate,yttropyrochlore and/or rutile class; and d) heating the glass inaccordance with holding curves determined by differential thermalanalysis until the crystalline phases have precipitated; whereby a glassceramic with a high modulus of elasticity greater than 110 GPa, athermal expansion coefficient (α₂₀₋₆₀₀) of 4 to 9×10⁻⁶ K⁻¹, a flexuralstrength of greater than 150 MPa, a surface roughness Ra of less than0.5 nm, and a Klc of 1.3 M Pam^(1/2) is formed.
 18. The method asdefined in claim 17, in which said crystallizable magnesium-containingaluminosilicate glass contains from 10 to 30 percent by weight of saidY₂O₃ and from 0 to 20 percent by weight of said Ln₂O₃.
 19. The method asdefined in claim 17, in which said crystallizable magnesium-containingaluminosilicate glass contains from 2 to 12 percent by weight of TiO₂,from 1 to 10 percent by weight of ZrO₂, and/or from 0 to 20 percent byweight of ZnO.
 20. The method as defined in claim 17, in which saidcrystallizable magnesium-containing aluminosilicate glass contains from0 to 5 percent by weight of CaO, from 0 to 5 percent by weight of SrO,and/or from 0 to 5 percent by weight of BaO.
 21. The method as definedin claim 17, in which said crystallizable magnesium-containingaluminosilicate glass contains at most 2 percent by weight of alkalimetal oxides.
 22. The method as defined in claim 17, in which saidcrystallizable magnesium-containing aluminosilicate glass contains lessthan 5 percent by weight of at least one transition metal oxide selectedfrom the group consisting of MnO₂, Fe₂O₃, NiO, CoO, Cr₂O₃, V₂O₅, MoO₃and WO₃.
 23. A magnetic storage disk comprising the glass ceramic asdefined in claim
 10. 24. A magneto-optical storage device comprising theglass ceramic as defined in claim
 10. 25. A mirror carrier comprisingthe glass ceramic as defined in claim 10.